This invention relates to waterborne nitrocellulose compositions which are particularly useful as coatings for printed metal foils.
Coating compositions containing resins and/or nitrocellulose are well known for use with a variety of packaging materials such as polymeric films, paper, and metal foils. See, for example, U.S. Pat. No. 3,556,826 which relates to nitrocellulose coating compositions for use with films used in wrapping odor sensitive products. Although many of these known compositions comprise similar components such as a resinous material, a solvent for the resin, and additives such as plasticizers, surfactants, slip aids and the like, the requirements for such a coating composition vary widely depending upon the particular intended use of the composition. The coating composition requirements are determined by such diverse factors as the production apparatus and process by which coating is to be accomplished, the substrate material to be coated, the desired characteristics of the resultant coated product, and even environmental regulations governing the permissible level of volatile organic components in the coating composition.
Thus, a coating composition which exhibits acceptable characteristics with one type of substrate may be totally useless with a different substrate. Similarly, the particular production or coating process by which the coating composition is applied to the substrate significantly determines the suitability of a particular coating composition.
Coating compositions for printed metal foils, such as printed aluminum foil, must satisfy a large number of production, aesthetic and environmental requirements. In terms of production requirements, such coating compositions must exhibit rapid forced drying and maintain rewet on the applicator roll. "Rewet" refers to the ability of the composition, which has been permitted to partially dry on the applicator roll, to be redispersed in an applicator bath containing the coating composition, upon restarting the production process.
In addition, the coating composition must impart certain physical properties to the resultant coated metal foil. In particular, the coated metal foil must possess a low static coefficient of friction and a low and uniform kinetic coefficient of friction, water resistance and surface mar resistance. Aesthetically, the dried coating composition must exhibit a high degree of gloss and clarity, and must not emit any objectional odors. Further, the coating composition must contain a minimal amount of organic solvents in order to comply with environmental regulations which strictly limit the amount of volatile organic components which may be present therein.
None of the known coating compositions for metal foils exhibit the required combination fo physical, aesthetic and environmental characteristics. In particular, known waterborne nitrocellulose coating compositions do not exhibit both rapid forced drying after application and rewet on the application roll, and do not impart a sufficiently low coefficient of friction or water resistance to the resultant coated metal foil.
It is, therefore, an object of the present invention to provide a waterborne nitrocellulose composition which does not suffer from the disadvantags of known coating compositions.
Another object of the present invention is to provide a nitrocellulose composition which is useful for coating printed metal foils.
A further object of the present invention is to provide a waterborne nitrocellulose composition which provides rewet on the applicator roll and imparts water resistance to a coated product.
A still further object of the present invention is to provide a coating composition which exhibits a high degree of gloss and clarity when applied to a substrate.
Yet a further object is to provide a nitrocellulose composition which dries rapidly.
A still further object is to provide a coating composition which imparts surface mar resistance to the coated substrate.
Another object is to provide a coating composition with a low volatile organic chemical content which meets air quality regulations.
Still another object is to provide a waterborne coating composition with a high glass transition temperature (T.sub.g) which allows desired properties to be achieved quickly after drying.
Additional objects and advantages of the present invention will be apparent to those skilled in the art by reference to the following description and the appended claims.